Early detection of stenosis is extremely important in order to prevent ischemic diseases such as myocardial infarction and cerebral infarction. For this reason, diagnosis assisting apparatuses equipped with blood vessel analyzing functions and blood vessel analyzing software have become available recently. For example, Patent Document 1 discloses a blood vessel stenosis rate analyzing system that detects stenosis sites of blood vessels by analyzing volume data obtained by CT imaging or MRI imaging, and calculates stenosis rates of blood vessels and area stenosis rates. This system displays three dimensional images of blood vessels generated by surface rendering or volume rendering, with colors added based on the calculated stenosis rates (refer particularly to paragraphs 0065 through 0072).